1. Field of the Invention
The invention relates to a thread-cutting, sleevelike screwed dowel.
2. Background Information
For fastening components to substrates, such as concrete, stone, brick masonry and the like, dowels are typically used. They have a sleevelike dowel body, which is anchored in a prepared bore in the substrate, and load engagement means, which for securing the component cooperate with the actual fastening means, such as a screw, bolt or the like. The load engagement means are typically female threads, which are provided in the inner wall of the sleevelike dowel body that is provided with a bore. However, other types of load engagement means can also be provided, such as bayonet mounts or similar positive-engagement connections. Another type of common load engagement means is insertable quick acting closures and the like. Anchoring the dowels in the substrate is typically done by radially spreading open a spreading region on the front end portion, in terms of the setting direction, of the dowel body. To that end, a typically conical expander body is displaced in the bore of the sleevelike dowel body, spreading the spreading regions radially apart. The expander body can be disposed inside the bore and driven forward into the bore. In so-called outer-cone dowels, the expander body is firmly clamped to the front end of the dowel body. For anchoring the dowel in the substrate, the usually metal sleevelike dowel body is driven over the metal expander body, the latter being braced on the bottom of the drilled hole. The spreading region is radially widened in the process.
The relative displacement between the sleevelike dowel body and the conical expander body is accomplished by means of axial chamfers, with which the cone is driven forward in the bore, or the dowel body is driven onto the expander body. Because the setting is done by hammering, the described expansion dowels are not suitable for every substrate. In brick masonry, for instance, the force of the axial blows damages the brick. After that, secure anchoring is longer possible. In other substrates as well, such as porous sandstone, the axial blows can cause damage to the expander body or to the sleevelike dowel body, with the result that the required load-bearing values are not attained. The non-positive anchoring of this type of dowel in the substrate by radial spreading apart of the spreading region of the dowel body is a disadvantage in many applications. Because of the relatively high spreading forces, minimum edge spacings and minimum axial spacings must be adhered to, if the security of the fastening point is to be assured. This often undesirably limits the possibilities for fastening components.
Since it is often absolutely necessary to provide fastening points at slight axial and peripheral spacings, in a second type of dowels a positive-engagement anchoring that is largely free of spreading pressure is provided. In so-called undercut dowels, “positive-engagement anchoring” means that expansion tabs provided on the dowel body are deployed in an undercut that is typically made in the vicinity of the base of the drilled hole. The undercut has to be made beforehand separately, with the aid of special undercutting devices in the drilled hole. This type of fastening technology is very time-consuming and expensive. It is therefore typically employed only for fastenings that are especially relevant to safety and especially for fastenings that must carry extremely heavy loads.
From the prior art, a dowel anchored by positive engagement is also known that makes do without the separate making of an undercut in the bore in the substrate. This dowel has a cylindrical dowel body, which is provided with an axial through bore. A female thread serves as the load engagement means. Cutting edges are disposed on the outer wall of the dowel body, winding helically around the dowel body in the manner of a steep thread. The steepness of the helically extending cutting edges is selected such that a cutting edge, over its entire length, wraps around less than 720° of the dowel body. In other words, the steep-threaded cutting edge, along its entire length, winds around the outer face of the dowel body in less than two windings. The cross section of the cutting edges is essentially triangular. A leading cutting edge in the setting direction forms a larger angle with a vertical to the dowel axis than the trailing cutting edge does. The steep leading edge, together with the great steepness of the steep-threaded cutting edges, is intended to assure that the dowel on being hammered into the drilled hole by axial blows will automatically dig into the wall of the hole and by cutting establish a positive engagement. The shallower trailing cutting edge is intended, by self-locking in the bore, to secure the dowel that is anchored by positive engagement. The setting operation for this known self-cutting dowel requires relatively strong axial blows. This limits the range of use of this dowel. For brick masonry or for less firm substrates, the dowel cannot be used, because the substrate can be damaged by the axial blows in the setting operation. The result would be that the required load-bearing values are unattainable. Securing the anchored dowel by self-locking in the cutting thread region makes it difficult to remove, should that become necessary. In particular, there is the risk that the wall of the bore will become damaged.